1. Technical Field
The present invention relates to electronic thermostats for heating/cooling systems and, more particularly, to thermostats employing a user-interactive microprocessor controlled by unique software which:
(1) operates the system in a cycling mode to optimize efficient energy utilization; and PA1 (2) facilitates installation and user interaction.
2. Discussion of the Prior Art
It has been recognized that more efficient use of a heating/cooling system is achieved by operating the system in a cycling mode in which relatively short on and off time periods are employed. In the description set forth herein, the "on" portion of each cycle is referred to as the on-time interval of the furnace (forced air or boiler) or air conditioner; the "off" portion is referred to herein as the "pause" interval. In some systems adaptive cycling is employed such that the relative and absolute durations of the on-time and pause intervals vary automatically as a function of the temperature in the temperature conditioned space. For example, in U.S. Pat. Nos. 4,199,023 (Phillips) and 4,460,123 (Beverly) the on-time and pause portions of a fixed duration cycle are varied as a function of temperature. In U.S. Pat. No. 4,356,962 (Levine '962) the on-time interval is changed for each cycle in accordance with the temperature change produced by the on-time interval of the previous cycle. In U.S. Pat. No. 4,408,711 (Levine '711) the on-time interval is adaptively modified to drive the conditioned space temperature to one limit of a dead zone which straddles the temperature set point; the pause interval is likewise modified to permit the temperature to return to the other limit of the dead zone.
The fixed duration, variable duty cycle approach employed by Phillips and Beverly is unable to maintain the temperature in the conditioned space precisely at the desired set point temperature. As a consequence, the temperature continuously varies back and forth between the limits of a dead zone range, typically on the order of three degrees. Apart from any discomfort which may be caused by this variation, the requirement of heating/cooling the space by these three degrees during each cycle results in inefficient use of the system. In the variable length cycles described in Levine '962 and Levine '711, the basic design philosophy requires temperature fluctuations over a dead zone range, thereby resulting in the same disadvantages brought about by fixed duration cycling.
The Phillips patent, noted above, expressly recognizes that the efficiency of fossil fuel furnaces depends somewhat on heat transfer exchange characteristic that are time dependent. Specifically, typical home furnaces are described as having time constants of about two minutes; that is, exhaust products reach approximately sixty-two percent of the final temperature in a two minute period. During the interval of the time constant the heat exchanger reaches a relatively high temperature and thereafter its ability to absorb heat decreases. It is also known that a higher percentage of the combustion heat is transferred to the heat exchanger when the walls are relatively cool. Therefore, Phillips reasons, the cycle on-time interval should last less than two minutes and the pause interval should be greater than one minute. In spite of this recognition, Phillips permits the on-time interval to exceed the two minute limit by as much as one hundred percent and permits pause intervals as short as one-half minute. While the facts on which Phillips bases system operation are valid, we have found that the facts have been improperly applied and, to some extent, are incomplete. Specifically, if the on-time interval is allowed to be shorter than the furnace or air conditioner time constant, the heat exchanger walls will not be permitted to reach the point of maximum absorption of thermal energy. Since considerable energy is required to impart thermal inertia to the furnace/air conditioner (i.e., to begin the process of thermal energy absorption by the heat exchanger walls), it is quite wasteful of energy to not permit the walls to reach maximum absorption once thermal inertia has been initiated. Moreover, if the furnace/air conditioner time constant is permitted to determine the maximum permissble on-time interval and minimum permissible pause interval, it is not hard to imagine temperature demands which could not be met by the system (i.e., the temperature in the conditioned space could not achieve setpoint). it is not surprising, then, that Phillips permits the on-time interval to exceed the stated maximum. Another factor leading to inefficiency in the Phillips system is the acceptance of the dead zone philosophy whereby each on-time interval is required to drive the temperature three degrees and each pause interval is required to permit the temperature to drift back by that same three degrees.
It is also known in the prior art that the compressor of an air conditioner unit can be severely damaged if, after being turned on and then turned off, the compressor is re-started before all the liquified refrigerant, that may have drained into the compressor, has vaporized. The problem is described, for example, in U.S. Pat. No. 4,453,590 (Holliday et al). In a cycling system this factor places a minimum on the pause interval, which minimum is considerably larger than the minimum pause interval permitted for a furnace. None of the prior art electronic thermostats are capable of accommodating these different minimum pause intervals in a common thermostat control for heating and cooling.
Apart from measured temperature conditions in the conditioned space, we have found that there are other characteristics which have an important bearing on the on-time and pause intervals required to effect optimum efficiency at any given time. For example, the time required to produce a given temperature change in the conditioned space may vary with outside temperature, the number of occupants in the space, activities within the space, the open/closed condition of doors and windows, etc. It is important, therefore, to know the Degree Transition Time of the space at all times so that the cycle on-time and pause intervals can be relatively selected at the onset of cycling for optimum efficiency. The parameter "Degree Transition Time" as used herein relates to the time required to raise the temperature in the conditioned space by one degree. The concept of degree transition time is described in U.S. Pat. No. 4,172,555 (Levine '555) in conjunction with predicting the time for energizing a heating/cooling system so that a system can be activated sufficiently in advance of a set point change to bring the temperature in the conditioned space to the set point at the required time. There has been no recognition in the prior art that the degree transition time parameter may be utilized to select cycle on-time and pause intervals for optimum fuel efficiency.
All prior art adaptive cycling thermostats are applicable to forced air systems but none relate to boiler-type heating systems which heat a conditioned space by radiation and/or convection. In fact, we have found that cycling is extremely effective in improving the efficiency of boiler type heating systems, particularly when cycling is utilized in accordance with the principles of the present invention. Surprisingly, these principles are applicable to controlling air conditioning systems, forced air heating systems, boiler type heating systems and any other temperature control system for a conditioned space.
There are numerous electronic thermostats known in the prior art, some of which are disclosed in U.S. Pat. Nos. 4,172,555 (Levine '555); 4,199,023 (Phillips); 4,206,872 (Levine '872); 4,356,962 (Levine '962); 4,408,711 (Levine '711); 4,460,123 (Beverly); 4,469,274 (Levine '274); and 4,473,183 (Kensinger et al). Some of these electronic thermostats include microprocessors controlled by specifically formulated software designed to effect intended heating/cooling system operation. Specifically, the aforementioned Beverly patent discloses a thermostat utilizing a microprocessor. All prior art electronic thermostats have been difficult for the average home owner to use in two distinct areas, namely:
(a) installation of the thermostat, particularly when an existing thermostat is replaced in a pre-existing heating/cooling system; and
(b) interaction with the thermostat to enter set points for various days and times.
With respect to the installation problem, the wires of an existing system are not labeled as to function, (i.e., burner, fan, air conditioner, voltage supply, etc.) and it is quite likely that the wires can be improperly connected, possibly causing damage to system equipment. Consequentially, the installation of prior art electronic thermostats, has required trained personnel.
With respect to ineracting with the system, there are prior art thermostats which permit different set points to be programmed into the system at different times. However, the set point entry procedure is so complicated that at least one consumer magazine has made a blanket recommendation to consumers that they should not purchase any of the programmable systems because of the programming difficulties.